diff options
Diffstat (limited to 'activerecord/lib/active_record/associations/preloader.rb')
| -rw-r--r-- | activerecord/lib/active_record/associations/preloader.rb | 108 |
1 files changed, 49 insertions, 59 deletions
diff --git a/activerecord/lib/active_record/associations/preloader.rb b/activerecord/lib/active_record/associations/preloader.rb index 83637a0409..4358f3b581 100644 --- a/activerecord/lib/active_record/associations/preloader.rb +++ b/activerecord/lib/active_record/associations/preloader.rb @@ -2,33 +2,42 @@ module ActiveRecord module Associations # Implements the details of eager loading of Active Record associations. # - # Note that 'eager loading' and 'preloading' are actually the same thing. - # However, there are two different eager loading strategies. + # Suppose that you have the following two Active Record models: # - # The first one is by using table joins. This was only strategy available - # prior to Rails 2.1. Suppose that you have an Author model with columns - # 'name' and 'age', and a Book model with columns 'name' and 'sales'. Using - # this strategy, Active Record would try to retrieve all data for an author - # and all of its books via a single query: + # class Author < ActiveRecord::Base + # # columns: name, age + # has_many :books + # end # - # SELECT * FROM authors - # LEFT OUTER JOIN books ON authors.id = books.author_id - # WHERE authors.name = 'Ken Akamatsu' + # class Book < ActiveRecord::Base + # # columns: title, sales + # end # - # However, this could result in many rows that contain redundant data. After - # having received the first row, we already have enough data to instantiate - # the Author object. In all subsequent rows, only the data for the joined - # 'books' table is useful; the joined 'authors' data is just redundant, and - # processing this redundant data takes memory and CPU time. The problem - # quickly becomes worse and worse as the level of eager loading increases - # (i.e. if Active Record is to eager load the associations' associations as - # well). + # When you load an author with all associated books Active Record will make + # multiple queries like this: + # + # Author.includes(:books).where(:name => ['bell hooks', 'Homer').to_a + # + # => SELECT `authors`.* FROM `authors` WHERE `name` IN ('bell hooks', 'Homer') + # => SELECT `books`.* FROM `books` WHERE `author_id` IN (2, 5) + # + # Active Record saves the ids of the records from the first query to use in + # the second. Depending on the number of associations involved there can be + # arbitrarily many SQL queries made. + # + # However, if there is a WHERE clause that spans across tables Active + # Record will fall back to a slightly more resource-intensive single query: + # + # Author.includes(:books).where(books: {title: 'Illiad'}).to_a + # => SELECT `authors`.`id` AS t0_r0, `authors`.`name` AS t0_r1, `authors`.`age` AS t0_r2, + # `books`.`id` AS t1_r0, `books`.`title` AS t1_r1, `books`.`sales` AS t1_r2 + # FROM `authors` + # LEFT OUTER JOIN `books` ON `authors`.`id` = `books`.`author_id` + # WHERE `books`.`title` = 'Illiad' + # + # This could result in many rows that contain redundant data and it performs poorly at scale + # and is therefore only used when necessary. # - # The second strategy is to use multiple database queries, one for each - # level of association. Since Rails 2.1, this is the default strategy. In - # situations where a table join is necessary (e.g. when the +:conditions+ - # option references an association's column), it will fallback to the table - # join strategy. class Preloader #:nodoc: extend ActiveSupport::Autoload @@ -80,7 +89,7 @@ module ActiveRecord # { author: :avatar } # [ :books, { author: :avatar } ] - NULL_RELATION = Struct.new(:values).new({}) + NULL_RELATION = Struct.new(:values, :bind_values).new({}, []) def preload(records, associations, preload_scope = nil) records = Array.wrap(records).compact.uniq @@ -112,13 +121,14 @@ module ActiveRecord end def preloaders_for_hash(association, records, scope) - parent, child = association.to_a.first # hash should only be of length 1 - - loaders = preloaders_for_one parent, records, scope + association.flat_map { |parent, child| + loaders = preloaders_for_one parent, records, scope - recs = loaders.flat_map(&:preloaded_records).uniq - loaders.concat Array.wrap(child).flat_map { |assoc| - preloaders_on assoc, recs, scope + recs = loaders.flat_map(&:preloaded_records).uniq + loaders.concat Array.wrap(child).flat_map { |assoc| + preloaders_on assoc, recs, scope + } + loaders } end @@ -140,36 +150,14 @@ module ActiveRecord end def grouped_records(association, records) - reflection_records = records_by_reflection(association, records) - - reflection_records.each_with_object({}) do |(reflection, r_records),h| - h[reflection] = r_records.group_by { |record| - association_klass(reflection, record) - } - end - end - - def records_by_reflection(association, records) - records.group_by do |record| - reflection = record.class.reflect_on_association(association) - - reflection || raise_config_error(record, association) - end - end - - def raise_config_error(record, association) - raise ActiveRecord::ConfigurationError, - "Association named '#{association}' was not found on #{record.class.name}; " \ - "perhaps you misspelled it?" - end - - def association_klass(reflection, record) - if reflection.macro == :belongs_to && reflection.options[:polymorphic] - klass = record.read_attribute(reflection.foreign_type.to_s) - klass && klass.constantize - else - reflection.klass + h = {} + records.each do |record| + next unless record + assoc = record.association(association) + klasses = h[assoc.reflection] ||= {} + (klasses[assoc.klass] ||= []) << record end + h end class AlreadyLoaded @@ -190,6 +178,7 @@ module ActiveRecord class NullPreloader def self.new(klass, owners, reflection, preload_scope); self; end def self.run(preloader); end + def self.preloaded_records; []; end end def preloader_for(reflection, owners, rhs_klass) @@ -198,6 +187,7 @@ module ActiveRecord if owners.first.association(reflection.name).loaded? return AlreadyLoaded end + reflection.check_preloadable! case reflection.macro when :has_many |